Working of Aircraft Heat Exchanger
A heat exchanger is a device that allows heat from a liquid or gas to pass onto a second fluid without the two fluids having to mix together or come into direct contact with one another. In simpler terms, the heat exchanger transfers heat without transferring the source of said heat. They are commonly used to cool hydraulics, ram air, auxiliary power units, gearboxes, and many other components found in an aircraft.
In power plants or engines, exhaust gases often release heat that wafts away into the open air. This is essentially wasted energy, something that a heat exchanger can reduce. Some heat will inevitably be lost, but the aircraft heat exchanger can minimize the amount. To achieve this, the heat exchanger is usually positioned inside the exhaust tailpipes or smokestacks. As the hot exhaust gases rise, they drift past copper fins with water flowing through them. The water in the fins absorbs the heat and carries it back into the plant where it can be recycled back into the engine or put to another use.
Though all heat exchangers do roughly the same thing, there are four common types of heat exchangers. They are: shell and tube heat exchanger, plate/fin heat exchanger, recuperator, and regenerator. In a shell and tube heat exchanger, one fluid flows through a set of metal tubes while the second fluid moves through a surrounding shell. As the fluids flow past each other, the heat is exchanged. Depending on the configuration, the fluids can flow in the same direction, opposite directions, or at right angles. Plate/fin heat exchangers operate in a similar way, but instead of tubes and a shell, they comprise many thin metals plates or fins with large surface areas to exchange heat more quickly.
The recuperator and regenerator are used to minimize heat loss from buildings, engines, and machines. A recuperator is used to capture heat that would otherwise be lost by channeling cold, incoming fluid in the opposite direction of warm, outgoing fluid. The two fluids flow through separate channels, never mixing. Because the fluids move in opposite directions, respirators are considered counterflow heat exchangers. A regenerator is similar to a recuperator, though the incoming and outgoing fluids flow through the same channel in opposite directions during different intervals. The warm fluid moves through the regenerator, leaving some of its heat behind. The cold fluid follows, picking up the heat left behind by the warm fluid.
Many heat exchangers are made from metal, which quickly absorbs and conducts heat. Despite this, heat exchangers can be made from a myriad of materials such as ceramics, composites (with their basis in metal or ceramic), and polymer plastics. Each of these materials has distinct advantages. For instance, ceramics are a great choice for high-temperature applications that would melt other materials like copper, iron, or steel. Ceramic heat exchangers can withstand temperatures in excess of 2000 degrees Fahrenheit. Ceramics are also resistant to corrosive and abrasive fluids of any temperature. Polymer plastics have the advantage of being lightweight, cost-effective, and the ability to be altered to have better thermal conductivity. The disadvantage of plastic is their mechanical weakness and proneness to degradation over time. Composite materials are highly effective as they combine the best features of their base materials, like the thermal conductivity of metal and the reduced weight of plastic.
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